Stopped-flow fluorometric study of the interaction of melittin with phospholipid bilayers: importance of the physical state of the bilayer and the acyl chain length

Abstract

Stopped-flow fluorometry has been employed to study the effects of melittin, the major protein component of bee venom, on dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) small unilamellar vesicles (SUVs) on the millisecond time scale, before melittin-induced vesicle fusion takes place. Use is made of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), which is an oriented fluorescent probe that anchors itself to the bilayer-water interface and is aligned parallel to the normal to the bilayer surface; its fluorescence anisotropy reports on the "fluidity" of the bilayer. For DMPC bilayers, melittin is found to decrease their fluidity only at their melting transition temperature. This perturbation appears to be exerted almost instantaneously on the millisecond time scale of the measurements, as deduced from the fact that its rate is comparable to that obtained by following the change in the fluorescence of the single tryptophan residue of melittin upon inserting itself into the bilayer. The perturbation is felt in the bilayer over a distance of at least 50 A, with measurements of transfer of electronic energy indicating that the protein is not sequestered in the neighborhood of TMA-DPH. The length of the acyl chains is found to be an important physical parameter in the melittin-membrane interaction: unlike the case of DMPC SUVs, melittin does not alter the fluidity of DPPC SUVs and has a considerably greater affinity for them. These results are discussed in terms of the concept of elastic distortion of the lipids, which results from a mismatch between the protein and the acyl chains that are attempting to accommodate it. Melittin is also found to cause a small (approximately 10%) enhancement in the total fluorescence intensity of TMA-DPH, which is interpreted as indicating a reduction in the degree of hydration of the bilayer.